Conventionally, there is a system in which a host side computer as a host apparatus and a device are connected through USB (Universal Serial Bus) communication (For example, refer to Patent Document 1). In such a system, the host and the device are connected through serial communication, and the system has various functions for enhancing user-friendliness, such as; a plug-and-play function (an automatic connection recognizing function), a hot insertion function (i.e., a function that enables connecting and disconnecting work under a condition where power supply is on), a power supply function from the host to the device, and so on.
In general, a USB cable to be used for USB communication has 4 wires in total; including a 5V power supply line (a V-BUS line), a GND (ground) line, and a D+ signal line and a D− signal line for data transfer. Then, a V-BUS signal transferred through the V-BUS line is always watched at the device side. That is to say; by always watching a transition condition of the V-BUS signal, it also becomes possible to detect an error of the USB communication. For example, in a system where a host apparatus of a V-BUS line does not intentionally turn off USB communication; if the USB cable gets disconnected due to some reason during USB communication, the V-BUS signal changes from ON condition to OFF condition so that the device recognizes that physical connection of the USB communication is broken and the error of the USB communication can also be detected.
[Patent Document 1]                Japanese Unexamined Patent Publication (Kokai) No. 2002-244775        